Veratrole, acylation

Bismuth chloride is a good catalyst for veratrole acylation by RCOC1 (entries 1,2), but the crude product may contain about 5 % of an impurity identified (GC-MS) as compound fi, which would arise from cleavage of an ether group by HC1. The reaction is more difficult with acid anhydrides (entries 4,5) relative to anisole (Table 4, entries 2,4). 

The activity of 42%STA/silica catalysts for the acylation of related aromatic reactants with iso-butyric anhydride was investigated. In the presence of anisole and veratrole, 100% anhydride conversion was observed, leading to the expected para-acylation products. No reaction was observed in the presence of chlorobenzene and other deactivated aromatic systems. 

On the other hand, a pure Eley-Rideal mechanism, in which the aromatic compound in the liquid phase reacts with the adsorbed acylating agent was first proposed by Venuto et alP1,22] and more recently by others.[23] However, for acylation reactions of polar substrates (anisole, veratrole), chemisorption of the latter must be taken into account in the kinetic law. A modification, the modified Eley-Rideal mechanism, has been proposed 114,24-26 an adsorbed molecule of acylating agent should react with a nonadsorbed aromatic substrate, within the porous volume of the catalyst. However, the substrate is also competitively adsorbed on the active sites of the zeolite, acting somehow as a poison of the acid sites. That is what we checked through different kinetic studies of various aromatic electrophilic substitution reactions.[24-26]... 

For example, in the acylation of veratrole with benzoic anydride,[14] following this mechanism, we assume that the veratrole chemisorption reduces the number of acid sites available for benzoic anydride, but that the reaction does not proceed between the two adsorbed species. Such an assumption leads to the corresponding initial rate equation as follows ... 

Acylation and alkylation of aromatic compounds. Benzophenones are obtained in good yield when a mixture of a phenolic ether and an alkoxybenzoic acid is treated with PPA prepared by stirring a mixture of 8 parts by weight of P2O5 and 5 parts by volume of 90% orthophosphoric acid (dl. 75) at 85° for 30 min. (Ayers and Denney ). Thus veratrole and vanillic acid afforded4-hydroxy-3,3, 4 -trimethoxybenzophenone. 

Rhodia has manufactured, selectively, acetoveratrole by acetylation of veratrole with acetic anhydride catalyzed by HBEA zeolite in the same process using fixed bed reactor technology. In this case, the best heterogeneous catalyst for acylation of veratrole is the HY zeolite. 

Unlike HBEA, it is possible to used powdered HY because even after reaction the filtration of HY CBV 720 sold by Zeolyst is possible and industrially realistic. In this case, the activity of the HY zeolite powder is very high, and as the ratio of veratrole/ zeolite in the reaction was very important, the recycling of the catalyst was not a priority. Eor a new plant dedicated to the acylation of the veratrole we can lower the investment because the recirculating fixed bed is not obligatory. Moreover, HY was cheaper than HBEA. 

Table 14.2 Acylation of veratrole with acetic anhydride over different zeolites.

Figure 14.3 Simplified flow chart of acylation of veratrole with acetic anhydride by HY powder.

Perot et al. [7] in collaboration with Rhodia have studied the deactivation of industrial catalysts HBEA and HY during the acylation of veratrole and anisole. After reaction, the spent catalysts were extracted with methylene chloride. This Soxhlet extraction makes possible the elimination of compounds that were not strongly adsorbed on the zeolites. The composition of the residue obtained after evaporation of methylene chloride was practically the same as that of the reaction mixture at the end of the experiment. By this extraction procedure, approximately 80% of the compounds remaining on the catalysts after reaction were recovered. After Soxhlet extraction, the catalyst samples were recovered and dissolved with hydrofluoric acid. The organic compounds released by the catalysts were extracted again by methylene chloride and, after evaporation of solvent, the residues contained di- and triketones as well as cyclization compounds, the structures of which are presented in Scheme 14.1. 

To confirm the inhibition effect of the adsorbed diketone on the acylation of veratrole by acetic anhydride over HBFA and HY zeolites, this product was added to reaction mixtures with both zeolites. Its presence caused a significant decrease in the yield in acylated products. For example, the addition of 3% (by weight with respect to veratrole) of diketone caused a loss in yield of more than 50% in both cases. 

For the acylation of anisole by acetic anhydride over HBFA and HY, the reasons for the deactivation accord with those already reported for the reaction with veratrole. Unfortunately, di- or triketones were not extractible and, also, deactivation of the... 

The acylation of 1,2-dimethoxyberizene or veratrole was carried out in the same conditions as those of anisole, either using acyl chlorides or acid anhydrides (eqn. 4) (Table 5). 

For this reaction, only zeolite catalysts were found to show some activity which are presented in Table 2. HY not only stand out as the best material, but in this case, the reaction was much faster, in accordance with the classical electrophilic substitution rules of more activated substrate. The Hb which was as active as the HY in the case of anisole is now much less active with veratrole (entry 3 and 4), underlining the effect of secondary parameters related to the catalyst itself. Such parameters includes, the structure of the zeolite, the size and the shape of the pores, the diffusion and chimisorption of both the substrate and the acylating agent. 

In the HZSM5, the intersection volume does not allow the intermediate to be formed and therefore, everything seems to happen at the outer surface of the catalyst. With this in mind, it is interesting to note that the yield obtained in the case of anisole and veratrole is the same indicating that the acylation of anisole on... 

The volume intersection of Hb is well suited for the acylation of anisole (compare Table 3 entry 2 with Table 4 entry 1) which is slightly superior to HY in term of activity, where the Weyland intermediate does not beneficiate from the confinement effect (ref. 5). On the other hand, it appears clearly that the acylated intermediate, in the case of veratrole can not be formed in the Hb, this intermediate being bigger than the allowed intersection volume. However, this situation is not encountered when HY are employed explaining the good catalytic results observed. 

A further antimony derivative, namely, bis-(para-methoxyphenyl) boryl hexachloroantimonate [prepared in situ from antimony pentachlo-ride and bis-(para-methoxyphenyl)boryl chloride] can be utilized (25% mol) in the acylation of anisole and veratrole with acetone acyl enolates in 52%-88% yield. The major advantage of the method resides in the possibility of performing Friedel-Crafts acylation at room temperature in a reaction medium that can be kept almost neutral throughout the reaction, acetone being the only co-product. 

The activity of some Y zeolites showing different Lewis and Bronsted acid sites density is still studied in the acylation of dimethoxyarenes (with particular attention to veratrole. Scheme 4.7) with different acyl chlorides as a function of zeolite acidity and the lipophilic nature of acyl chlorides. Because of the particularly soft reaction conditions, namely, 65°C and 1 h reaction, 3,4-dimethoxyphenyl ketones 12 are the sole isomers recognized in the reaction mixture. Results of the catalytic tests (yield of compounds 12) confirm that the best catalyst is Y(14), characterized by an optimum ratio between Lewis and Bronsted acidity (medium-strength acid sites density for Lewis and Bronsted acid = 17.0 and 11.0 mmol x g- py. 

The remarkable effect of pore dimensions and structural properties on zeolite efficiency is further confirmed in the acylation of veratrole... 

Highly selective acylation at the 4-position is achieved by the reaction of propanoic anhydride with anisole (68% yield) and veratrole (52% yield) in the presence of clayzic at 90°C for 10 h in nifrobenzene. The cafalyfic activity depends on the proper balance of Bronsfed and Lewis acid sites present in the clayzic. 

Breda, A., Signoretto, M., Ghedini, E., Pinna, R, and Cruciani, G. 2006. Acylation of veratrole over promoted SZ/MCM-41 catalysts influence of metal promotion. Appl. Catal. A Gen. 308 216-222. 

Acylation of anisole and veratrole with Complex process using AICI3 Zeolite Y and P Simplified process in a fixed-bed reactor [66]... 

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